A quantitative structure-activity relationship (QSAR) study dealing with narcotic agonists and antagonists is in progress. The experimentally determined receptor binding constant, log P, pka, and C13 NMR chemical shift values will be correlated with the twitch-response of the longitudinal muscle of the guinea pig ileum. The Hansch multiple parameter analysis and the Free-Wilson additivity model will be used to correlate the above data with the biological activity.